The invention relates to a process for producing precision castings from a melt by means of a metallic casting wheel having an annular distribution channel and having a plurality of interchangeable casting moulds, each having at least one filling opening, the melt quantity per casting operation being chosen such that the casting moulds and the distribution channel are filled with the melt upon the rotation of the casting wheel about its axis, in such a way that after the solidification of the melt the precision castings are held together by a ring of the casting material which is formed in the distribution channel and are removed from the casting wheel together with the casting moulds, whereupon the precision castings are separated from the ring and the material of the ring is fed to a recycling process.
It concerns in particular, but not exclusively, the production of precision castings composed of titanium-containing materials.
A process, disclosed by EP 0 686 443 A1, deals primarily with the selection of particular mould materials which have an influence on the casting and solidification behaviour of melts composed of titanium-containing materials such as
The invention also embraces such casting materials, but is not restricted to them. Examples of other suitable materials are nickel-based alloys, high-temperature nickel aluminides, in particular materials which are highly reactive at their casting temperature, which also includes the above casting materials.
Possible applications are in the field of internal combustion engines, e.g. for parts which move in oscillating fashion, such as valves, connecting rods and piston pins, where the mass, noise and thermal behaviour plays a role. But possible applications are also in the field of rotating machines, such as turbine wheels, turbine blades, compressor wheels and parts thereof, that is to say all mass-produced articles, where the production costs, precision and compliance with all product parameters play a crucial role. Further interesting fields of application are biomedical prostheses, such as, for example, implants, sporting and leisure articles, tools and the like.
If such materials have simple geometries, that is to say are designed for example rotationally symmetrically, such as valves, it is possible to employ separable metallic moulds which can be reused as often as desired and which merely have to be opened, but not destroyed, to demould the castings.
In the process disclosed by EP 0 686 443 A1, a plurality of rings of castings are produced in each case in separable moulds around a central runner, and these castings are also joined between the rings by the material solidified in the runner to form a tree or cluster of castings. There is a further consideration: most of the above-described materials are hard and brittle at room temperature, and solid but still ductile at temperatures between about 200xc2x0 C. and 300xc2x0 C. Upon demoulding at room temperature, however, brittle fractures may occur, leading to rejects.
The known apparatus is not intended for ceramic casting moulds which can be used only once and have to be destroyed to demould complicated castings.
DE 19 84 678 B2 and the corresponding EP 0 992 305 A1 likewise disclose the production of precision castings by casting melt into separated metallic casting moulds, which are arranged in radial orientation within an axially separated metallic casting wheel. The arrangement of the casting moulds within the casting wheel results in the formation, between its radial walls, of a distribution channel which directly communicates with the mould cavities of the casting moulds. The melt quantity is chosen per casting operation such that after the solidification of the melt a ring of casting material is also present in the distribution channel, from which ring the castings protrude radially and with which ring they form a one-piece cast body. After the opening of the casting wheel and removal of the cast body, the castings are demoulded by removal of casting-mould parts, reusable as often as desired, and are separated from the ring. With this casting technology, which is applicable only to nondestructively demouldable castings, such as, for example, engine valves, the melt does not come into contact with ceramic or oxidic materials at any point, so that the material quantities which do not belong to the castings, and this includes in particular the ring, can be melted down again and recast. These quantities of expensive materials, the so-called xe2x80x9ccirculating materialxe2x80x9d, amount to about 50 to 70% of the total material quantity.
In the case of the production of complicated, in particular, undercut, precision castings, which can be demoulded only with the destruction of the casting moulds, casting technology has retained the practice of using new, ceramic or oxidic casting moulds for each casting operation. For economical fabrication to be possible here, such casting moulds are connected by ceramic or oxidic casting channels, resulting in whole clusters of castings, the entire surfaces of which have come into contact with the ceramic or oxidic cluster moulds and thereby been contaminated. Although this is still tolerable for the precision castings themselves, it makes the reuse or recycling of the remaining material quantities (the circulating material) problematical, since the contaminating components accumulate therein. It was therefore decided to allow such material quantities to be reused as xe2x80x9ccirculating materialxe2x80x9d only once. This increases the cost of the production process enormously, particularly in the case of expensive casting materials, such as, for example, the materials based on titanium, in particular the very expensive titanium aluminides, described at the outset.
Owing to its high affinity for oxygen, titanium especially, given the material-dependent processing temperatures (it cannot be demoulded at room temperature owing to brittleness), has the property of absorbing oxygen and oxygen compounds from the ceramic mould materials and even reacting with these mould materials. The contact with ceramic materials causes a marked reduction in the ductility of the casting material and makes reuse of the xe2x80x9ccirculating materialxe2x80x9d or xe2x80x9creturn scrapxe2x80x9d more difficult, it only being possible consequently to add this material again in small quantities in precision casting processes.
The object on which the invention is based therefore is to specify a process and an apparatus which enable the use, with high productivity, of ceramic casting moulds usable only once, without the circulating material being contaminated to an unacceptable level and the reuse, recycling, of this circulating material being intolerably restricted. These requirements are in a way diametrically opposed.
The stated object is achieved according to the invention for the process specified at the outset in that the casting moulds are selected from a ceramic material and are attached in a positive-locking and interchangeable manner to the casting wheel and so as to protrude therefrom.
The spatial orientation of the casting moulds and their mould cavities may be radial, oblique or tangential to the casting wheel. It is also possible for the casting moulds to have more than one, e.g. two, filling openings. Casting moulds having a filling opening at each of the two ends of the mould cavity may be used especially for casting turbine blades. The axis of rotation of the casting wheel does not have to be horizontal either, but may also be oriented at an angle to the horizontal or even vertically. In the last-mentioned case, one side of the casting wheel should then preferably be closed.
By means of the invention, the stated object is achieved in its entirety, in particular the use, with high productivity, of ceramic casting moulds usable only once is enabled, without the circulating material being contaminated to an unacceptable level and the reuse, recycling, of this circulating material being intolerably restricted. The diametrically opposed requirements are thus simultaneously achieved.
The melt comes into contact with ceramic or oxidic materials only inside the casting moulds, i.e. only once, but this is not the case of the material quantities not belonging to the castings, and this includes in particular the ring, it being possible for these material quantities to be melted down again and recast. These quantities of expensive materials, the so-called xe2x80x9ccirculating materialxe2x80x9d, which amounts to about 50 to 70% of the total material quantity, can thus be reused without significant restrictions. There is no increasing contamination due to oxygen and/or oxides, nor is it necessary to limit the number of precision castings to so-called clusters. The invention enables, for example, the simultaneous production of about 50 turbocharger wheels in one process cycle.
This enables the economical production of complicated, in particular, undercut, precision castings which can be demoulded only with the destruction of the casting moulds. As a result, the cost of the production process is reduced enormously, particularly in the case of expensive casting materials, such as, for example, the materials based on titanium, in particular the very expensive titanium aluminides, described at the outset. The ductility of the casting material is preserved and enables multiple reuse of the xe2x80x9ccirculating materialxe2x80x9d or of the xe2x80x9creturn scrapxe2x80x9d, which can consequently be added again in large quantities in precision casting processes.
In further refinements of the process according to the invention, it is particularly advantageous if: the casting wheel used is one having two wheel rings which abut against each other at an annular parting line in which recesses for inserting the casting moulds are situated, if after the insertion of the casting moulds the wheel rings are moved together axially with the formation of the distribution channel, and if after the casting operation the wheel rings are moved apart axially with the release of the ring of casting material, together with the casting moulds and the precision castings.
The invention also relates to an apparatus for producing precision castings from a melt by means of a metallic casting wheel having an annular distribution channel and having a plurality of interchangeable casting moulds, each having at least one filling opening, it being possible for the casting moulds and the distribution channel to be filled with the melt upon the rotation of the casting wheel about its axis, in such a way that after the solidification of the melt the precision castings are held together by a ring of the casting material which is formed in the distribution channel and can be removed from the casting wheel together with the casting moulds.
To achieve the same object, such an apparatus is characterised according to the invention in that the casting moulds are selected from a ceramic material and are attached in a positive-locking and interchangeable manner to the casting wheel and so as to protrude therefrom.
In further refinements of the apparatus according to the invention, it is particularly advantageous ifxe2x80x94either individually or in combination:
the casting wheel has two wheel rings which abut against each other at an annular parting line in which recesses for inserting the casting moulds are situated, if after the insertion of the casting moulds the wheel rings can be moved together axially with the formation of the distribution channel, and if after the casting operation the wheel rings can be moved apart axially with the release of the ring of casting material, together with the casting moulds and the precision castings;
the casting moulds each have a flanged edge which surrounds their filling openings and is insertable in a positive-locking manner, at least substantially parallel to the axis of the casting wheel, into the complementary recesses in the parting line of the casting wheel;
the wheel rings are provided at their outsides with holding rings which engage behind the flanged edges of the casting moulds on in each case part of their periphery;
at least one of the holding rings can be detached from the associated wheel ring and fitted with the casting moulds outside the casting wheel;
at least one of the holding rings is subdivided into sectors;
there are arranged in at least one of the wheel rings casting gates which connect the distribution channel to the filling openings of the casting moulds;
the casting moulds are surrounded by an annular catching channel;
the catching channel has an annular disc, a cylindrical body and a radially inwardly directed annular flange, and if the catching channel is fastened to one of the wheel rings;
the respective other wheel ring has fastened to it a further annular disc which, when the casting wheel is closed, at least substantially closes an opening in the catching channel;
the casting wheel is assigned a loading device, by means of which preheated casting moulds are insertable into the casting wheel;
the loading device is designed to receive at least one of the holding rings; and/or if
the loading device is designed to receive sectors of at least one of the holding rings.
The subject-matter of the invention is preferably suitable for the centrifugal casting of precision castings.
An exemplary embodiment of the subject-matter of the invention and the way in which it works are explained in more detail below with reference to FIGS. 1 to 6.